Powder comprising silica-coated zinc oxide, organic polymer composition containing the powder and shaped article thereof

ABSTRACT

A powder comprising silica-coated zinc oxide fine particles in which the surface of each particle is coated with silica, wherein large particles of 5 μm or more account for 0.1 mass % or less. A powder comprising surface-hydrophobicized silica-coated zinc oxide fine particles in which the silica-coated zinc oxide fine particles whose surfaces have been coated with silica are further treated with a hydrophobicity-imparting agent, wherein large particles of 5 μm or more account for 0.1 mass % or less.

FIELD OF THE INVENTION

The present invention relates to zinc oxide employed in organic polymercompositions, rubber products, paper, cosmetics, paints, printing ink,etc., and more particularly to powder containing silica-coated zincoxide particles having a smaller number of large particles, to organicpolymer compositions containing such powder, and to shaped productsformed from the compositions.

BACKGROUND ART

Zinc oxide, also called zinc flower, has long been known as a whitepigment. Zinc oxide is endowed with the following optical properties:When zinc oxide is reduced to fine particles having a diameterapproximately half the wavelength of visible light, the particles allowvisible light to pass therethrough, because the scattering effect of thezinc oxide particles deteriorates considerably, and selectively absorbultraviolet rays by virtue of the excellent ultraviolet absorbing effectof zinc oxide.

In relation to ultraviolet absorbers making use of such zinc oxideparticles, Japanese Patent Application Laid-Open (kokai) No. 5-171130discloses a resin molded product in which zinc oxide fine powder havinga particle size of 0.1 μm or less is incorporated into a transparentresin. Japanese Patent Application Laid-Open (kokai) Nos. 5-295141 and11-302015 disclose zinc oxide fine particles which are coated with asilicon-containing compound in order to prevent possible impairment ofweather resistance of products containing the fine particles which wouldotherwise be attributable to the photocatalytic action of zinc oxide andto improve the dispersibility of the fine particles in a resin.

Japanese Patent No. 2501663 (International Publication WO90/06974)discloses a method of encapsulating a zinc oxide composition for pigmentuse in a shell formed by causing water-insoluble metallic soap to bedeposited on the pigment. The method includes the steps of adding, to aslurry of the pigment-use zinc oxide composition, a water-soluble alkalimetal salt of saturated or unsaturated monocarboxylic acid having 7 to22 carbon atoms, and a water-soluble metal salt composed of a cationmoiety of metal selected from among groups IB, II, III, IV, V, VIB,VIIB, and VIII of the periodic table and an inorganic anion moietyselected from a nitrate ion, a sulfate ion and a halogen ion, whereby insitu formation and deposition of water-insoluble metallic soap of thementioned saturated or unsaturated monocarboxylic acid is achieved.

Meanwhile, methods using a solvent (such as water or an organic solvent)in surface treatment require steps including filtration and drying forthe solvent and, therefore, maldistribution of the surface treatmentagent that is deposited during drying or coalescence of powder particlestends to occur. Thus, this method is accompanied by a shortcoming thatgood dispersion of the coated zinc oxide particles is difficult toattain.

In uses related to apparel and packaging materials, transparency,weather resistance, flexibility, or like properties are frequentlydemanded and, in such cases, demand has arisen for thin films or thinfibers having a high level of UV shielding ability.

Conventional zinc oxide particles are not satisfactory in terms ofphotocatalytic effect or an effect preventing release of zinc ions, andthis is true even in the case of surface-treated zinc oxide particles,because the surface treatment is insufficient. Thus, in suchconventional zinc oxide particles, degradation of organic materialscannot be avoided and the durability of resultant products is low, inpractice.

For example, as polyesters and polyamides are molded and processed athigh temperature, use of organic UV absorbers therewith is difficult. Aconceivable approach to avoid this problem is use of, as an inorganic UVabsorber, zinc oxide particles. However, the mentioned resins readilydecompose, and in addition, are endowed with properties that permitdegradation as a result of interaction with zinc ions, so that providingpractical, durable compositions is difficult.

Also, in cases where fibers obtained by spinning a compositioncontaining conventional zinc oxide particles are dyed, there arisesanother problem in that release of zinc ions into the dye solutioncannot be avoided.

Moreover, since conventional coated zinc oxide particles permit theco-presence of large particles, processing of a resin compositioncontaining such conventional particles involves the following problems:in formation of thin fibers such as multifilaments, breakage of threadfrequently occurs; in the formation of a very thin inflation film,puncture occurs; and in tape formation, the stretch factor is limited.

The present invention is directed to provision of a powder containingfinely divided, specific silica-coated zinc oxide particles containing asmaller number of large particles which ensure facilitated shaping ofthin film, thin fiber, or similar products which are free from impairedweather resistance which would otherwise be attributable tophotocatalytic action and which are endowed with sufficient UV shieldingability; organic polymer compositions containing such powder; and shapedproducts formed from the compositions.

Furthermore, the present invention is directed to provision of thepowder, organic polymer compositions comprising such powder; and shapedproducts formed from the compositions, which are free from a bleed-outphenomenon, unlike an organic UV absorber, and have good durabilityagainst washing.

SUMMARY OF THE INVENTION

The present inventors have carried out extensive research in an attemptto attain the above objectives, and have found that use of silica-coatedzinc oxide powder containing a smaller number of large particles;specifically, use of such a powder in which large particles having asize of 5 μm or more are contained in an amount of 0.1% by mass or less,in combination with a thermoplastic resin enables facilitated shaping ofthin film, thin fiber, or similar products which are free from impairedweather resistance, which would otherwise be attributable tophotocatalytic action, and which are endowed with sufficient UVshielding ability, leading to completion of the present invention.

Accordingly, the present invention comprises the following:

(1) A powder comprising silica-coated zinc oxide fine particles in whichthe surface of each particle is coated with silica, wherein largeparticles of 5 μm or more account for 0.1 mass % or less.

(2) A powder comprising surface-hydrophobicized silica-coated zinc oxidefine particles in which the silica-coated zinc oxide fine particles,whose surfaces have been coated with silica, are further treated with ahydrophobicity-imparting agent, wherein large particles of 5 μm or moreaccount for 0.1 mass % or less.

(3) The powder as recited in (2), wherein the hydrophobicity-impartingagent is one or more members selected from the group consisting ofsilicone oils, alkoxysilanes, silane coupling agents, and higher fattyacid salts.

(4) The powder as recited in any of (1) to (3), wherein thesilica-coated zinc oxide fine particles have silica coating of 0.5 to100 nm in thickness.

(5) The powder as recited in any of (1) to (4), wherein thesilica-coated zinc oxide fine particles have an average primary particlesize of 1 to 200 nm.

(6) The powder as recited in (2) or (3), wherein thesurface-hydrophobicized, silica-coated zinc oxide fine particles have anaverage primary particle size of 5 to 120 nm and a silica-film thicknessof 0.5 to 25 nm.

(7) The powder as recited in any of (1) to (6), wherein ratio I ofinfrared absorption peak intensity of silica film of the silica-coatedzinc oxide fine particles at 1150 to 1250 cm⁻¹ to that at 1000 to 1100cm⁻¹ as determined on an infrared absorption spectrum is 0.2 or more(I=I1/I2; wherein I1 denotes absorption peak intensity at 1150 to 1250cm⁻¹ and I2 denotes absorption peak intensity at 1000 to 1100 cm⁻¹), andthe silica film has a refractive index of 1.435 or more.

(8) The powder as recited in any of (1) to (7), wherein the powderexhibits a photocatalytic activity of 60 Pa/min or less as measuredthrough the tetralin auto-oxidation method.

(9) The powder as recited in any of (1) to (8), wherein the powderexhibits a dye color fading rate (ΔABS₄₉₀/hour) of 0.1 or less asmeasured through the sunset yellow method.

(10) The powder as recited in any of (1) to (9), wherein the powderexhibits an organic UV absorber decomposition rate (ΔABS₃₄₀/hour) of0.01 or less as measured through the Parasol method.

(11) The powder as recited in any of (1) to (10), wherein the powderexhibits a percent organic UV absorber decomposition of 5% or less asmeasured through the ethylhexyl p-methoxycinnamate method.

(12) The powder comprising silica-coated zinc oxide fine particles asrecited in any one of (1) to (11), which includes titanium oxide.

(13) The powder comprising silica-coated zinc oxide fine particles asrecited in (12), wherein titanium oxide in an amount of 2 parts by massto 5 parts by mass is further included based on zinc oxide at 10 partsby mass.

(14) The powder comprising silica-coated zinc oxide fine particles asrecited in (12) or (13), wherein at least one part of titanium oxide iscoated with silica.

(15) The powder comprising silica-coated zinc oxide fine particles asrecited in any one of (12) to (14), wherein the titanium oxide containsmixed crystal having a titanium-oxygen-silicon bond in its primaryparticles.

(16) The powder comprising silica-coated zinc oxide fine particles asrecited in (15), wherein when the BET specific surface area of titaniumoxide is represented by “A m²/g” and the SiO₂ content is represented by“B mass %”, the ratio of B/A is 0.02 to 0.5.

(17) The powder comprising silica-coated zinc oxide fine particles asrecited in (15) or (16), wherein BET specific surface area of thetitanium oxide is 10 to 200 m²/g.

(18) The powder comprising silica-coated zinc oxide fine particles asrecited in any one of (15) to (17), wherein the average primary particlesize of titanium oxide is 0.008 Am to 0.15 μm.

(19) The powder comprising silica-coated zinc oxide fine particles asrecited in any one of (15) to (18), wherein the titanium oxide has core(a nucleus)/shell (a husk) structure, wherein the core is a TiO₂-richstructure and the shell is an SiO₂-rich structure.

(20) An organic polymer composition containing a powder comprisingsilica-coated zinc oxide fine particles as recited in any one of (1) to(19), and a thermoplastic resin.

(21) An organic polymer composition consisting essentially of a powdercomprising silica-coated zinc oxide fine particles as recited in any oneof (1) to (19), and a thermoplastic resin.

(22) The organic polymer composition as recited in (20) or (21), whereinthe thermoplastic resin is selected from the group consisting ofpolyethylenes, polypropylenes, polystyrenes, polyamides, polyesters, andpolycarbonates.

(23) A shape-imparted product of an organic polymer composition asrecited in any one of (20) to (22).

(24) The shape-imparted product as recited in (23), which is selectedfrom the group consisting of fibers, yarns, films, tapes, hollowproducts, and multi-layer structures.

(25) An object comprising a shape-imparted product as recited in (23) or(24) and selected from the group consisting of building materials forinterior furnishings and exterior finish, machinery, exterior andinterior decor materials for automobiles, glass products, electricappliances, agricultural materials, electronic apparatus, tools,tableware, bath products, toiletry products, furniture, clothing, wovenfabrics, non-woven fabrics, cloth products, leather products, paperproducts, sporting goods, futon, containers, eyeglasses, signboards,piping, wiring, brackets, sanitary materials, automobile parts, outdoorgoods such as tents, panty hose, socks, gloves, and masks.

(26) The cosmetic material including the powder comprising silica-coatedzinc oxide fine particles as recited in any one of (1) to (19).

MODES FOR CARRYING OUT THE INVENTION

The ZnO-containing powder of the present invention is preferably apowder, containing silica-coated zinc oxide fine particles whosesurfaces are coated with silica, in which large particles having a sizeof 5 μm or more are present in an amount of 0.1% by mass or less.Another type of a preferred powder according to the present invention isa powder containing surface-hydrophobicized silica-coated zinc oxidefine particles in which the silica-coated zinc oxide fine particleswhose surfaces have been coated with silica are further treated with ahydrophobicity-imparting agent, wherein large particles of 5 μm or moreaccount for 0.1 mass % or less.

A process for producing the powder of the present invention will next bedescribed in detail.

No limitation is imposed on the process for producing the startingmaterial of the powder of the present invention containing silica-coatedzinc oxide; i.e., silica-coated zinc oxide fine particles whose surfacesare coated with silica. An exemplary method that may be employed isdisclosed in International Publication WO98/47476 (hereinafter may bereferred to as “the present method”.)

Specifically, the production process of the silica-coated zinc oxidefine particles according to the present method includes a step ofbringing a specific composition for forming silica coating into contactwith raw material zinc oxide particles whose primary particles have anaverage particle size of 5 nm to 200 nm, wherein the composition forforming silica coating contains at least the following components:

1) silicic acid containing neither an organic group nor a halogen, or aprecursor capable of producing such silicic acid,

2) water,

3) an alkali, and

4) an organic solvent, and preferably the water/organic solvent ratio byvolume falls within a range of 0.1 to 10, and the silicon content fallswithin a range of 0.0001 to 5 mol/L, whereby surfaces of the zinc oxideparticles are selectively coated with a dense silica coating. Thethus-formed silica film satisfactorily covers complicated surfaces ofthe base material; i.e., zinc oxide particles, and, even when thethickness of the film is as thin as 0.5 nm, excellent coverage and highshielding against photocatalytic activity can be ensured. In addition,as a silica coating having an extremely low alkali metal content can berealized, even under high-temperature high-humidity conditions, thesilica film does not suffer the problem of dissolving to thereby affectthe properties of silica-coated zinc oxide.

Within the context of the present method, the silicic acid to be usedfor preparing a composition for forming silica film collectively refersto orthosilicic acid and its polymers such as metasilicic acid,mesosilicic acid, mesotrisilicic acid, and mesotetrasilicic acid. Theseare described, for example, in Encyclopaedia Chimica (Kyoritsu ShuppanK.K., published on Mar. 15, 1969, seventh print) under the heading“silicic acid.” The silicic acid contains neither an organic group nor ahalogen.

The silicic acid to be employed in the present method may be obtained byadding water, an alkali, and an organic solvent to tetraalkoxysilane(Si(OR)₄, wherein R is a hydrocarbon group, in particular a C1-C6aliphatic group); more specifically, to a precursor capable of producingsilicic acid, such as tetramethoxysilane, tetraethoxysilane,tetra-n-propoxysilane, tetraisopropoxysilane, or tetra-n-butoxysilane;and the mixture is stirred to thereby cause hydrolysis. This method isadvantageous in that handling and operation are easy and practical. Ofthe mentioned materials, tetramethoxysilane is preferred.

In this connection, compounds represented by the following formula:X_(n)Si(OH)_(4-n)(wherein X denotes a hydrocarbon group, a halogen, or hydrogen; and n isan integer of 1, 2, or 3), which have a hydrophobic moiety such as ahydrocarbon group, a halogen, or hydrogen, do not fall within thedefinition of the precursor capable of producing silicic acid.Therefore, trialkoxyalkylsilane, dialkoxyalkyldialkylsilane,trialkoxysilane, dialkoxysilane, and analogous substances are notsuitable precursors for the purposes of the present invention.

Alternative methods for producing a composition containing silicic acidinclude: addition of water, an alkali, and an organic solvent totetrahalosilane for hydrolysis; addition of an alkali and an organicsolvent to water glass; and application of water glass onto cationexchange resin, followed by addition of an alkali and an organicsolvent. No particular limitation is imposed on tetraalkoxysilane,tetrahalosilane, and water glass, which serve as the raw materials forpreparing silicic acid, and those which are widely used for industrialpurposes or as reagents may be employed. However, higher purity is morepreferred. According to the present invention, the composition forforming a silica coating may contain unreacted substances remaining fromthe raw materials for producing silicic acid.

No particular limitation is imposed on the amount of the silicacontained in the composition for forming silica coating. Preferably, thesilicon content is 0.0001 to 5 mol/L, more preferably 0.001 to 5 mol/L.Silicon contents of lower than 0.0001 mol/L are not practical, becausethe silica film formation rate will be too slow, whereas siliconcontents exceeding 5 mol/L are detrimental, because silica particles maybe generated in the composition, without forming silica film.

The silicon content can be calculated on the basis of the amount of araw material (e.g., tetraethoxysilane) for producing silicic acid.Alternatively, the silicon content may be determined through atomicabsorption spectroscopy of the resultant composition for forming silicacoating. In the analysis, the analysis target may be a spectrum ofsilicon at a wavelength of 251.6 nm, and a flame of acetylene/dinitrogenoxide may be employed.

No particular limitation is imposed on water to be employed forpreparing the composition for forming silica film. However, if foreignmatter is present in water, it may migrate into the final products as animpurity, and therefore, such foreign particles are preferably removedbeforehand through, for example, filtration.

The water which is employed for producing the composition for formingsilica coating is preferably used at a water/organic solvent ratio (byvolume) of 0.1 to 10. If the water/organic solvent ratio (by volume)falls outside this range, a film cannot be formed, or the film formationrate drops significantly. A more preferred range for the water/organicsolvent ratio (by volume) is 0.1 to 0.5. So long as the water/organicsolvent ratio (by volume) falls within this range of 0.1 to 0.5, noparticular limitation is imposed on the species of the alkali to beemployed. However, if the water/organic solvent ratio (by volume) is 0.5or more, film formation is preferably carried out by use of an alkalicontaining no alkali metal; for example, by use of ammonia, ammoniumhydrogencarbonate, or ammonium carbonate.

In the present method, examples of the alkali used in the compositionfor forming silica film include, but are not limited to, inorganicalkalis such as ammonia, sodium hydroxide, and potassium hydroxide;inorganic alkali salts such as ammonium carbonate, ammoniumhydrogencarbonate, sodium carbonate, and sodium hydrogencarbonate;organic alkalis such as monomethylamine, dimethylamine, trimethylamine,monoethylamine, diethylamine, triethylamine, pyridine, aniline, choline,tetramethylammonium hydroxide, and guanidine; and organic alkali saltssuch as ammonium formate, ammonium acetate, monomethylammonium formate,dimethylammonium acetate, pyridin lactate, guanidinoacetic acid, andanilin acetate.

Of the above compounds, the following compounds are particularlypreferred from the viewpoint of better regulation of reaction rate:ammonia, ammonium carbonate, ammonium hydrogencarbonate, ammoniumformate, ammonium acetate, sodium carbonate, sodium hydrogencarbonate,etc. In the composition for forming a silica film, one or more alkalispecies selected from among the above group may be used in combination.

In the present method, no particular limitation is imposed on the purityof the alkali. Although purity levels widely accepted for industrialuses or as reagents are applicable, a higher purity is more preferred.

An effective way of increasing the silica coating formation rate is toraise the temperature at which the film is formed. In this case, thealkali and the organic solvent are preferably chosen from among speciesthat are not easily volatilized or decomposed at the film formationtemperature.

In the present method, a very small amount of alkali may suffice forforming a film and, thus, in the case where sodium carbonate is employedas the alkali, film can be formed by addition of an amount as small as0.002 mol/L. Of course, a large amount; for example, as large as 1mol/L, of sodium carbonate may be added. However, addition of a solidalkali in a large amount that exceeds the solubility limit is notpreferred, because the alkali would migrate as an impurity into themetal oxide powder.

Through use of an alkali which does not contain an alkali metal as aprimary component, there can be created silica-coated metal oxideparticles having a low alkali metal content. From the viewpoints of filmformation rate and readily removal of residue, the most preferred ofthese are ammonia, ammonium carbonate, and ammonium hydrogencarbonate.

In the present invention, preferably, the organic solvent contained in acomposition for forming silica coating is selected from among thosecapable of forming a homogeneous solution of the composition. Examplesof such organic solvents include alcohols such as methanol, ethanol,propanol, and pentanol; ethers/acetals such as tetrahydrofuran and1,4-dioxane; aldehydes such as acetaldehyde; ketones such as acetone,diacetone alcohol, and methyl ethyl ketone; and polyhydric alcoholderivatives such as ethylene glycol, propylene glycol, and diethyleneglycol. Of these solvents, alcohols are preferred and ethanol is morepreferred, from the viewpoint of ease of regulating the reaction rate.As the organic solvent, one species may be chosen, or two or morespecies may be chosen for use as a mixture.

No particular limitation is imposed on the purity of the organic solventto be contained in the composition for forming silica coating, andorganic solvents which are widely used for industrial purposes or asreagents may be employed. However, a higher purity is more preferred.

The composition for forming a silica coating can be prepared through agenerally performed solution preparation method. For example,predetermined amounts of an alkali and water are added to an organicsolvent, and after stirring, tetraethoxysilane is added, followed byfurther stirring. The sequential order of addition may be changed, andregardless of the order of addition, coating can be satisfactorilyformed. From the viewpoint of control of reaction, upon mixing water andtetraethoxysilane, both are preferably diluted with an organic solvent.

The thus-prepared composition for forming a silica coating is a stablecomposition, and until it is brought into contact with metal oxideparticles such as zinc oxide particles, substantially no coating ordeposition occurs. When the composition contacts metal oxide particles,silica is selectively deposited on the surfaces of the metal oxideparticles, to thereby form silica coating. As used herein, the word“selectively” is used to refer to the case where film formation proceedsas silica is deposited on the surfaces of the metal oxide, withoutinducing formation of silica particles occurring in association withuniform nucleus formation in a solution, whereby stoichiometricalcontrol of silica film thickness and the silica content of thesilica-coated metal oxide is possible.

No particular limitation is imposed on the method of producing zincoxide which serves as the raw material of the silica-coated zinc oxidefine particles, and any appropriate method may be used. Thus, there maybe employed zinc oxide obtained through evaporation oxidation of crudeelectrolyzed zinc bar; zinc hydroxide obtained by neutralizing anaqueous solution of a water-soluble salt such as zinc sulfate or zincchloride; fired products obtained through firing zinc carbinate, zincsulfate, or zinc oxalate; or mixtures of any of these. Other types ofzinc oxide which may be employed include zinc oxide doped with ahetero-element such as Fe, Co, Al, Sn, or Sb; and mixed crystal oxidesor complex oxides containing zinc oxide as a primary component, and alsocontaining crystalline or non-crystalline oxide of an element selectedfrom among Si, Al, Fe, Co, Zr, Ce, Sn, Sb, and like elements. A zincoxide which forms little aggregation is preferred from the viewpoint ofcontrol of secondary particle size.

The average primary particle size of zinc oxide particles which serve asraw material in the present method is preferably 1 nm to 200 nm, morepreferably 5 nm to 120 nm. The average secondary particle size ispreferably 0.5 μm or less.

According to the present method, zinc oxide particles serving as astarting material are immersed in a composition for forming silicacoating, and the resultant system is maintained at a predeterminedtemperature, whereby the surfaces of the zinc oxide particles areselectively coated with silica, resulting in formation of silicacoating. Specifically, the silica coating may be formed through a methodin which, firstly, a composition for forming silica coating is preparedand, then, zinc oxide particles serving as a starting material aresupplied for forming silica coating; or alternatively, zinc oxideparticles serving as a starting material are suspended in a solvent inadvance and, subsequently, other starting materials are added to therebyform a composition for forming silica coating, and then a silica coatingis formed. That is, no particular limitation is imposed on the startingmaterials of the coating composition or on the sequential order in whichthe starting zinc oxide particles are added; a silica coating can beformed regardless of the order of addition.

In particular, a preferred method is performed in such a manner thatfirstly a suspension containing starting zinc oxide particles, water, anorganic solvent, and an alkali is prepared, and then tetraalkoxysilanediluted with an organic solvent is added dropwise at a constant rate ofaddition. This process enables provision of silica coating of improveddensity, resulting in realization of a continuous step which isindustrially beneficial.

Growth of silica film proceeds on the basis of the selective depositionof silica on the surfaces of zinc oxide particles. Therefore, the longerthe film formation time, the thicker the film thickness. Of course, ifsilicic acid contained in the film forming composition is mostlyconsumed for forming coatings, the film forming rate decreases. However,by sequentially adding silicic acid in consumed amounts, silica coatingcan be formed continuously at a practical speed. Particularly throughthe process including the steps of maintaining starting zinc oxideparticles in a coating composition to which a certain amount of silicicacid has been added, the amount corresponding to a silica coatingthickness of interest; forming silica coating to thereby consume silicicacid; removing the produced silica-coated zinc-oxide fine particles tothe outside of the system; and adding silicic acid in an amountcorresponding to the consumed amount is added, the composition can againbe used in the subsequent coating step for the starting zinc oxideparticles, attaining a continuous process which is very economical andhighly productive.

For example, in the case where tetraalkoxysilane diluted with an organicsolvent is added dropwise at a constant rate to a suspension containingstarting zinc oxide particles, water, and an organic solvent, completeconsumption of tetraalkoxysilane and formation of dense silica coatinghaving a film thickness of interest can be attained by usingtetraalkoxysilane in an amount corresponding to the silica coatingthickness of interest and diluting it with an organic solvent, and theresultant solution is added dropwise at a constant rate that iscommensurate with the hydrolysis rate. Subsequently, the generatedsilica-coated zinc oxide fine particles are taken out of the reactionsystem, yielding a product of high purity in which virtually nounreacted tetraalkoxysilane remains. Needless to say, the solvent fromwhich the silica-coated zinc oxide fine particles has been removed canbe used again in a recycling manner for the next run of film formation,to thereby realize an economical, highly productive process.

No particular limitation is imposed on the temperature at which silicacoating is formed; the temperature is preferably 10-100° C., morepreferably 20-50° C. The higher the temperature, the higher the filmformation speed. However, when the temperature is excessively high,components of the composition evaporate, and the compositionalproportions of the solution cannot be maintained, whereas when thetemperature is excessively low, the film formation speed isimpractically low.

The pH of the composition for forming silica coating should be alkalineduring film formation, in order to attain a satisfactory density ofcoating. As the solubility of zinc oxide may vary in a pH-dependentmanner, the pH of the composition for forming a silica coating ispreferably controlled by modifying the amount of the alkali added.However, in such a case, as the amount of alkali changes, the hydrolysisrate of tetraalkoxysilane or a similar material varies, and therefore,film formation temperature or water content of the coating compositionmust be regulated so that an appropriate hydrolysis rate is attained.

After the zinc oxide particles are coated with silica, liquid/solidseparation is performed, whereby silica-coated zinc oxide fine particlescan be isolated. Isolation may be effected through a customaryseparation method, such as filtration, centrifugal sedimentation, orcentrifugal separation.

After the step of solid/liquid separation, a drying step is performed,whereby silica-coated zinc oxide fine particles having a low watercontent are produced. Drying may be performed through a conventionaldrying method, such as natural drying, hot-air application, vacuumdrying, or spray drying. Firing of the silica-coated zinc oxide fineparticles is not particularly required. However, they may be used afterfiring.

The silica coating of the silica-coated zinc oxide fine particlesproduced through the present method has dense coating, and thus isadvantageous for use in practice. Within the context of the presentinvention, the term “dense, means that the formed silica film has highdensity and is uniform without pinholes or cracks. The term “practical”means that strong bonding (—Si—O—Zn— bonding) between silica and zincoxide (serving as a substrate) prevents defoliation of the coating or alike phenomenon, whereby the physical properties of silica-coated zincoxide tend to be consistent.

The silica-coated zinc oxide fine particles produced through the presentmethod are preferably surface-hydrophobicized silica-coated zinc oxidefine particles obtained through subjecting the particles to surfacetreatment with a hydrophobicity-imparting agent.

The surface treatment of silica-coated zinc oxide fine particles with ahydrophobicity-imparting agent may be performed by use of a knownmethod. In the present method, silica-coated zinc oxide particles may bedirectly hydrophobicized by use of the dry method or the spray method.The dry method may proceed as follows: To silica-coated untrafine mixedcrystal oxide particles which are being stirred in a mixing device (suchas a V-shape mixer or a Henschel mixer), a hydrophobicity-impartingagent or an organic solution of a hydrophobicity-imparting agent isadded by way of spraying or similar means, and mixing is furtherperformed to thereby allow the agent to deposited onto the surfaces ofthe powder particles. The resultant particles are dried and, whennecessary, heat may be applied to strengthen the bonding. Alternatively,when a spray method is employed, a hydrophobicity-imparting agent or asolution of a hydrophobicity-imparting agent is sprayed ontosilica-coated zinc oxide particles heated to high temperature, wherebythe surface coating can be effected.

According to the wet method, silica-coated ultrafine mixed crystal oxideis dispersed in water or an organic solvent, or in a mixture of waterand an organic solvent, and to the resultant dispersion, ahydrophobicity-imparting agent (or a solution containing ahydrophobicity-imparting agent) and a reaction catalyst are added,followed by stirring and then surface treatment. In this case, if adrying step is performed after solid/liquid separation,surface-hydrophobicized silica-coated zinc oxide fine powder can beobtained. Drying may be performed through a conventional drying method,such as natural drying, hot-air application, vacuum drying, or spraydrying.

As the above-described silica-coated zinc oxide fine powder orsurface-hydrophobicized silica-coated zinc oxide fine powder undergoescohesion of particles during the process of drying or firing, a step forreducing large particles must be performed. In order to reduce thenumber of large particles, a dry-format classification is preferred. Forexample, precision classification can be carried out by use of aturbo-classifier produced by Nisshin Engineering K.K. or similar means.Intensive milling attained by use of a jet mill may be effective forreducing the level of aggregation of particles. However, such intensivemilling may cause partial breakage of silica coating or create newsurfaces (i.e., zinc oxide surfaces) as a result of milling ofsurface-treated products of large zinc oxide particles. These are notpreferred because processability and weather resistance of organicpolymer composition containing such intensively milled particles aredeteriorated. A wet-format stationary classification employing a solventis not preferred, either, because of possible re-aggregation which mayarise during a solid/liquid separation step or a drying step afterclassification.

Examples of the hydrophobicity-imparting agent used in the presentmethod include, but are not limited to, higher fatty acids such aswaxes, higher fatty acid triglycerides, higher fatty acids, higher fattyacid polyvalent metal salts, and polyvalent metal higher fatty sulfatesalts; higher alcohols or derivatives thereof; organic fluorinecompounds such as perfluorinated or partial-fluorinated higher fattyacids and higher alcohols; and organic silicon compounds such assilicone oils, organic alkoxysilanes, organic chlorosilanes, andsilazanes. Among them, higher fatty acid polyvalent metal salts,silicone oils, silane coupling agents, and alkoxysilanes are preferablyemployed.

Examples of the silicone oils used in the present method include, butare not limited to, dimethylpolysiloxanes, methylhydrogenpolysiloxanes,methylphenylpolysiloxanes, and cyclic polydimethylsiloxanes.Alternatively, modified silicone oils such as alkyl-modified,polyether-modified, amino-modified, mercapto-modified, epoxy-modified,and fluorine-modified silicone oils may also be employed.

Examples of the chlorosilanes used in the present method include, butare not limited to, trimethylchlorosilane, dimethyldichlorosilane,methyltrichlorosilane, methyldichlorosilane, dimethylvinylchlorosilane,methylvinyldichlorosilane, triphenylchlorosilane,methyldiphenylchlorosilane, diphenyldichlorosilane,methylphenyldichlorosilane, and phenyltrichlorosilane.

Examples of the silazanes used in the present method include, but arenot limited to, hexamethyldisilazane, N,N′-bis(trimethylsilyl)urea,N-trimethylsilylacetamide, dimethyltrimethylsilylamine,diethyltrimethylsilylamine, and trimethylsilylimidazole.

Examples of the organic alkoxysilanes used in the present methodinclude, but are not limited to, silane coupling agents such asvinyltrichlorosilane, vinyltris(β-methoxyethoxy)silane,vinyltrimethoxysilane, vinyltriethoxysilane,γ-(methacryloyloxypropyl)trimethoxysilane,β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane,γ-glycidyloxypropyltrimethoxysilane,γ-glycidoxypropylmethyldiethoxysilane,N-β(aminoethyl)γ-aminopropyltrimethoxysilane,N-β(aminoethyl)γ-aminopropylmethyldiethoxysilane,γ-aminopropyltriethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane,γ-mercaptopropyltrimethoxysilane, and γ-chloropropyltrimethoxysilane;methyltrimethoxysilane; dimethyldimethoxysilane; trimethylmethoxysilane;methyltriethoxysilane; dimethyldiethoxysilane; trimethylethoxysilane;methyldimethoxysilane; methyldiethoxysilane; dimethylethoxysilane;dimethylvinylmethoxysilane; dimethylvinylethoxysilane;phenyltrimethoxysilane; phenyltriethoxysilane; diphenyldimethoxysilane;and diphenyldiethoxysilane. Alternatively,. alkoxysilanes having aperfluorinated or partial-fluorinated alkyl group may also be used.

In particular, alkoxysilanes represented by the following formula arepreferably used:Formula: R¹(R² _(n)) SiX_(3-n)(wherein R¹ is a C1-C4 alkyl or phenyl, R² is hydrogen, a C1-C4 alkyl orphenyl, X is a C1-C4 alkoxyl, and n is an integer of 0 to 2).

The coating amount of the hydrophobicity-imparting agent is equal to orgreater than the minimum coating amount required for thehydrophobicity-imparting agent to achieve complete coverage over thesurfaces of the silica-coated zinc oxide particles serving as rawmaterial. This amount is calculated from the following equation.Minimum coating amount (g)={Mass of the silica-coated ultrafine mixed crystal oxide (g)}×{Specificsurface area (m²/g)}÷{minimum area covered by thehydrophobicity-imparting agent (m²)}

Use of excessive amounts of the hydrophobicity-imparting agent permitsdeposition of the agent on portions other than the surfaces of thesilica-coated ultrafine mixed crystal oxide particles, and thus is noteconomical. Although the amount is not universally determined, becauseit varies depending on the molecular weight of thehydrophobicity-imparting agent and the specific surface area of thesilica-coated untrafine mixed crystal oxide particles, in general, theamount preferably falls within a range of 0.1 to 30 mass % inclusive,more preferably 1 to 20 mass % inclusive. Both ranges of less than 0.1mass % and those more than 30 mass % are not preferred, because in theformer case sufficient hydrophobicity cannot be obtained, and in thelatter case, although sufficient hydrophobicity can be obtained, a UVshielding effect is lowered because of reduction in the amount of zincoxide per unit weight of particles.

The thickness of the silica coating of the silica-coated zinc oxide fineparticles which are used in the present method is 0.5 to 100 nm,preferably 1.0 to 50 nm, more preferably 1.5 to 25 nm. Silica coatingthicknesses of less than 0.5 nm cannot provide satisfactory shieldingagainst photocatalytic action and, in addition, there may be cases wherestable organic polymer compositions, shape-imparted products, orstructures fail to be obtained. Also, silica coating thicknesses inexcess of 100 nm are not preferred, because there may be cases where theresultant organic polymer compositions, shape-imparted products, orstructures fail to be endowed with sufficient UV shielding ability. Inthis connection, the thickness of silica coating is determined on thebasis of images obtained through transmission electron microscopy.

The silica-coated zinc oxide fine particles which are used in thepresent method have an average primary particle size of 1 to 200 nm,preferably 5 to 120 nm. Average primary particle sizes falling outsidethe above respective ranges are less preferred, in view that theyinvolve an increased tendency of failure in obtaining organic polymercompositions, shape-imparted products, or structures having high UVshielding ability.

As used herein, the “primary particles” are those defined in “Powders”authored by Kiichiro KUBO, et al. (pp. 56-66, published 1979).

The silica-coated zinc oxide fine particles obtained through theabove-described method have such a characteristic that ratio I ofinfrared absorption peak absorbance of silica film of the silica-coatedzinc oxide fine particles at 1150 to 1250 cm⁻¹ to that at 1000 to 1100cm⁻¹ as determined on an infrared absorption spectrum is 0.2 or more,preferably 0.3 or more, more preferably 0.4 or more (I=I1/I2; wherein I1denotes absorption peak intensity at 1150 to 1250 cm⁻¹ and I2 denotesabsorption peak intensity at 1000 to 1100 cm⁻¹, after subtraction of thebase line value). A transmission infrared absorption spectrum of thesilica film of silica-coated zinc oxide can be obtained by use of theKBr powder method.

Generally, silica coating obtained through firing by use of, forexample, the sol-gel method, or through CVD exhibits ratio I ofabsorption peak intensity at 1150 to 1250 cm⁻¹ to that at 1000 to 1100cm⁻¹ of lower than 0.2. Variation of this value is generally accepted tobe attributable to changes in chemical bonds or modifications offunctional groups, which alter characteristics of silica film in termsof hydrophilicity and oil absorption.

The silica layer of the silica-coated zinc oxide fine particles whichare used in the present method preferably has a refractive index of1.435 or more, more preferably 1.440 or more. Refractive indices lessthan 1.435 are less preferred, in view that density of the layerdecreases. Silica film which has been obtained through a typical sol-gelmethod but which has not been fired has a refractive index less than1.435. Such a film, having a low density, is not practically used. Ingeneral, the density of silica film is known to be positively correlatedwith the refractive index thereof (e.g., C. Jeffery Brinker, Sol-GelScience, 581-583, Academic Press (1990)).

As used herein, within the context of the present method, the term“dense” means that the formed silica film has high density and isuniform without pinholes or cracks. The term “practical” means thatstrong bonding (—Si—O—Zn— bonding) between silica and zinc oxide(serving as a substrate) prevents defoliation of the coating or a likephenomenon, whereby physical properties of silica-coated zinc oxide tendto be consistent.

The refractive index is determined by use of a silica film which isformed on a silicon wafer which has been simultaneously immersed in acomposition for forming silica coating upon synthesis of silica-coatedzinc oxide. In other words, the same silica film as formed on thesurfaces of zinc oxide particles is considered to be formed on thesilicon wafer. The refractive index of the silica film formed on thesilicon wafer can be determined by use of an ellipsometer (LASSERELLIPSOMETER ESM-1A, product of ULVAC).

The silica-coated zinc oxide fine particles which are used in thepresent method exhibit a photocatalytic activity of 60 Pa/min or less,preferably 50 Pa/min or less, as measured through the tetralinauto-oxidation method (the initial oxygen consumption). When thephotocatalytic activity as measured through the tetralin auto-oxidationmethod exceeds 60 Pa/min, the effect of suppressing photocatalyticactivity cannot be fully attained, possibly by failing to attainsatisfactory durability, which is not preferred.

The tetralin auto-oxidation method is described by Manabu KIYONO in“Titanium oxide—Physical Properties and Application Techniques,”published by Giho-do, pp. 196-197, 1991. Determination conditionsinclude: temperature; 40° C., tetralin; 20 mL, and zinc oxide; 0.02 g.

The photocatalytic activity of the silica-coated zinc oxide fineparticles which are used in the present method is measured through thesunset yellow method (dye color fading rate), the Parasol 1789 method,or the ethylhexyl p-methoxycinnamate method, which are described inExamples.

The dye color fading rate (ΔABS490/hour) of the silica-coated zinc oxidefine particles which are used in the present invention is preferably 0.1or less, more preferably 0.05 or less, as measured through the sunsetyellow method. When the dye color fading rate exceeds 0.1, the effect ofsuppressing photocatalytic activity cannot be fully attained, possiblyby failing to attain satisfactory durability, which is not preferred.

The organic UV absorber decomposition (UV absorber: Parasol 1789) rateof the silica-coated zinc oxide fine particles which are used in thepresent invention is preferably 0.02 or less, more preferably 0.01 orless, as measured through the Parasol 1789 method. When the organic Uvabsorber decomposition rate as measured through the Parasol 1789 methodexceeds 0.02, the effect of suppressing photocatalytic activity cannotbe fully attained, possibly failing to attain satisfactory durability,which is not preferred.

The percent organic UV absorber decomposition (UV absorber: ethylhexylp-methoxycinnamate) of the silica-coated zinc oxide fine particles whichare used in the present invention is preferably 5% or less, morepreferably 3% or less, as measured through the ethylhexylp-methoxycinnamate method. When the percent organic UV absorberdecomposition as measured through the ethylhexyl p-methoxycinnamatemethod exceeds 5%, the effect of suppressing photocatalytic activitycannot be fully attained, possibly by failing to attain satisfactorydurability, which is not preferred.

The amount of large particles (5 μm or more) contained in thesilica-coated zinc oxide powder is determined in the following manner.

When a sample is a powder containing silica-coated zinc oxide fineparticles obtained by coating the surfaces of zinc oxide particles withsilica, the sample (20 g) is accurately weighed, and is then placed inpurified water (1,800 ml) at room temperature, followed by thoroughstirring. A proper amount (10 ml) of a dispersant, such as a 10% aqueoussolution of sodium hexametaphosphate, is added to the above mixture,followed by stirring. After completion of stirring, the mixture issubjected to ultrasonic dispersion for 10 minutes. The ultrasonicdispersion can be performed by use of an ultrasonic homogenizer (e.g.,US-300T, product of Nippon Seiki Seisakusho, output: 300 W, oscillationfrequency: 20 kHz). Subsequently, the suspension is poured onto aprecision micro-mesh sieve having a mesh-size of 5 μm set on a particleclassifier of Yokohama Rika (model PS-80). Subsequently, wet precisionclassification is performed by means of an ultrasonic vibrator, anelectromagnetic vibrator, and a suction pump, which are incorporated inthe apparatus. After completion of the classification, water in awashing bottle is jetted to the sieve in order to collect the powderremaining on the sieve, and the powder is then placed in a glasscontainer together with purified water. The glass container is placed ina 110° C. drying apparatus so as to evaporate water. The remainingresidue is collected and weighed. The weight ratio of the residue to theoriginal sample (20 g) represents the amount of large particles having aparticle size of 5 μm or more.

When a sample is a powder containing hydrophobicized silica-coated zincoxide fine particles obtained by coating the surfaces of zinc oxideparticles with silica and then surface-treating the particles by use ofa hydrophobicity-imparting agent, in the above-described classificationoperation, wet precision classification can be performed by using,instead of water, an equi-volume mixture solvent of water and methanolas the solvent other than the dispersant. After completion of theclassification, the powder remaining on the sieve is dried by air, andis then placed in a 110° C. drying apparatus. The remaining residue iscollected and weighed. The weight ratio of the residue to the originalsample (20 g) represents the amount of large particles having a particlesize of 5 μm or more.

The thus-obtained amount of large particles (5 μm or more) contained inthe powder containing the silica-coated zinc oxide fine particles ispreferably 0.1 mass % or less, more preferably 0.05 mass % or less.

When the amount of large particles (5 μm or more) exceeds 0.1 mass %,incorporation of the powder in an amount for substantially expressing UVshielding effect raises the following problems: in formation of thinfibers such as multifilaments, breakage of thread frequently occurs; information of very thin inflation film, puncture occurs; and in tapeformation, stretch factor is limited.

Use of the aforementioned silica-coated zinc oxide powder in combinationwith a thermoplastic resin facilitates shaping of thin film, thin fiber,or similar products which are free from impaired weather resistancewhich would otherwise be attributable to photocatalytic action and whichare endowed with sufficient UV shielding ability.

The powder comprising silica-coated zinc oxide fine particles of thepresent invention may contain titanium oxide. UV shielding ability canbe increased by containing titanium oxide particles.

In this case, it is preferable to contain the titanium oxide in anamount of 2 parts by mass to 5 parts by mass based on 10 parts by massof zinc oxide, more preferably 2.5 to 5 parts by mass, and mostpreferably 3 to 5 parts by mass. It is preferable to contain thetitanium oxide in an amount of 2 parts or more by mass in order toincrease UV shielding ability by means of titanium oxide mixture.However, when the titanium oxide content is more than 5 parts by mass,there may be an unfavorable case in which the titanium oxide causesunacceptable whiteness and lack of transparency.

Regarding the titanium oxide to be contained, a silica-coated titaniumoxide produced by coating in the same method as silica-coated zinc oxidementioned above, is preferred.

The production method of titanium oxide as a starting material of thesilica-coated titanium oxide is not particularly limited and any methodmay be used. A titanium oxide produced by any production method such ashigh-temperature vapor phase oxidation of TiCl₄, vapor phase hydrolysisof TiCl₄, a sulfuric acid process and a chlorine process may be used.With respect to the crystal form of titanium oxide, any of amorphous,rutile, anatase, and brookite may be used and a mixture thereof may alsobe used. In view of control of the secondary particle size, the titaniumoxide is preferably reduced in impurities as much as possible and, morepreferably, is reduced in coagulation.

In addition, as the titanium oxide to be contained, the ultrafinemixed-crystal oxide particles containing, in primary particles, mixedcrystals having a titanium-oxygen-silicon bond, may be used. Noparticular limitations are imposed on the method for producing theultrafine mixed-crystal oxide particles comprising, in primaryparticles, mixed crystals having a titanium-oxygen-silicon bond, and theultrafine particles may be produced through, for example, the methoddescribed in International Publication WO01/56930.

For the ultrafine mixed-crystal oxide particles containing, in primaryparticles, mixed crystals having a titanium-oxygen-silicon bond, theratio B/A may be 0.02 to 0.5, preferably 0.05 to 0.3, when the BETspecific surface area is represented by “A m²/g” and the SiO₂ content isrepresented by “B mass %”. When the ratio of B/A is less than 0.02, thedispersion of the ultrafine mixed-crystal oxide particles isinsufficient in an organic polymer composition and the resultantshape-imparted product exhibits poor weather resistance. When the ratioof B/A exceeds 0.5, the amount of SiO₂ in the surfaces of the particlesincreases, and the improvement effect of UV shielding ability islowered, which is not preferred.

The ultrafine mixed-crystal oxide particles containing, in primaryparticles, mixed-crystals having a titanium-oxygen-silicon bond, mayhave a BET specific surface area of 10 to 200 m²/g, and preferably 15 to100 m²/g. When the BET specific surface area exceeds 200 m²/g,difficulty is encountered in producing the particles efficiently, andwhen the BET specific surface area is less than 10 m²/g, the improvementeffect of UV shielding ability is lowered, this is not preferred.

In addition, an average primary particle size is generally 0.008 μm to0.15 μm, and preferably 0.015 μm to 0.1 μm. When the average primaryparticle size of the ultrafine mixed-crystal oxide particles containing,in primary particles, mixed-crystals having a titanium-oxygen-siliconbond is less than 0.008 μm, difficulty is encountered in producing theparticles efficiently, and when the average primary particle size ismore than 0.15 μm, the improvement effect of UV shielding ability islowered, which is not preferred.

Each of the ultrafine mixed-crystal oxide particles employed in thepresent invention preferably has a core (a nucleus)/shell (a husk)structure, in which the core is TiO₂-rich structure and the shell isSiO₂-rich structure. In this case, SiO₂ phase is typically supported ona portion of the surface of the particle. The SiO₂ phase supported onthe surface may assume a dot-like or island-like form (i.e., adiscontinuous form), or a strand-like, net-like, or porous form (i.e., acontinuous form). Alternatively, the SiO₂ phase supported on the surfacemay assume both a continuous form and a discontinuous form.

The powder comprising silica-coated zinc oxide fine particles of thepresent invention can be used as a cosmetic material.

In this case, the cosmetic material can be an arbitrary agent type,having a W/O emulsion form, an O/W emulsion form, a liquid form, a solidform, or a gel form, by using the conventional producing method andcommon raw materials.

For example, an extender pigment (e.g., mica, talc, kaolin, calciumcarbonate, magnesium carbonate, silisic acid anhydride, aluminum oxide,barium sulfate), a white pigment (e.g., titanium dioxide, zinc oxide), acolor pigment (e.g., red oxide of iron, yellow oxide of iron, blackoxide of iron, chromium oxide, ultramarine, iron blue, carbon black), aspherical powder (e.g., nylon powder, polymethyl mathacrylate powder),an oil portion (liquid petrolatum, squalane, castor oil, glyceryldiisostearate, glyceryl triisosterate, glyceryl tri-2-ethylhexanoate,isopropyl myristate, dimethylpolysiloxane, methylphenylpolysiloxane,petrolatum, diisostearyl maleate and purified lanolin), an organic UVabsorber (benzophenone type, salicylic acid type, dibenzoylmethane typeand urocanic acid type), an existing emulsifier, or an existingantiphlogistic ingredient, not particularly limited, may be used incombination or may be mixed.

Because the powder comprising silica-coated zinc oxide fine particles ofthe present invention has a high photocatalytic activity-suppressingeffect, even if used together with organic UV absorber, thedecomposition of the absorber is suppressed and it is used as thecosmetic material having a high and long-life UV shielding ability.

When an antioxidant as a substance (substrate) having anoxidation-inhibiting activity is used in combination in the cosmeticmaterial of the present invention, the amount of free radicals generatedby ultraviolet rays can be suppressed further, whereby thephotocatalytic activity of silica-coated titanium oxide or silica-coatedzinc oxide can be more reduced and therefore, a safe cosmetic materialhaving a remarkably excellent preparation stability and a lowphototoxicity can be obtained.

The amount of the powder comprising silica-coated zinc oxide fineparticles blended in the cosmetic material of the present invention ispreferably from 5 to 25% by mass, more preferably from 5 to 20% by mass,based on the cosmetic material.

If the amount of the above combination is less than 5% by mass, theultraviolet shielding effect is insufficient. If it is more than 25% bymass, the cosmetic material produces a poor feel during cosmetic use,such as a pale finish in make-up and a rough or creaky feel, and this isnot preferred.

An organic polymer composition including the powder comprisingsilica-coated zinc oxide fine particles of the present invention hasexcellent processability and moldabilty. Furthermore, the shape-impartedproducts produced from the composition exhibit superior weatherresistance. In particular, shape-imparted products such as stockings,socks and underwear, etc., which are produced by molding/processingextra-fine fibers such as for example a multi-filament, and packagematerials and agriculture materials, which are produced bymolding/processing extra-thin film, exhibit superior shapability,processability and weather resistance.

Furthermore, the molding/processing products in the present invention donot exhibit a bleed out phenomenon which organic UV absorbers do.Therefore, stockings, socks and underwear, etc. have excellentdurability against wash, even after being shaped.

The silica-coated zinc oxide powder of the present invention is blendedwith a thermoplastic resin, to thereby provide an organic polymercomposition. Examples of the thermoplastic resin include, but are notlimited to, polyethylene, polypropylene, polystyrene, polyethyleneterephthalate, AS resins, ABS resins, AES resins, polyvinylidenechloride, methacrylic resins, polyvinyl chloride, polyamides,polycarbonates, polyallyl esters, polyimides, polyacetals, polyetherketones, polyether sulfones, polyphenyl oxides, and polyphenylenesulfides. The amount of silica-coated zinc oxide powder contained in theorganic polymer composition is generally 0.01 to 80 mass %, preferably0.1 to 50 mass %, more preferably 1 to 20 mass %. When a master batch isproduced, the amount is generally 1 to 80 mass %, preferably 10 to 40mass %.

To the thermoplastic resin, generally employed coloring agents,fluorescent agents, and additives may be added in accordance with need.Examples of the additives include antioxidants, anti-aging agents,UV-absorbers, lubricants, antistatic agents, surfactants, fillers (e.g.,calcium carbonate and talc), plasticizers, stabilizers, blowing agents,expanding agents, electroconductive powder, electroconductive shortfiber, deodorizing agents, softening agents, thickeners,viscosity-reducing agents, diluents, water-repellent agents,oil-repellent agents, cross-linking agents, and curing agents.

However, when thin film, thin fiber, or a similar material is producedfrom the composition, these additives, coloring agents, and fluorescentagents preferably contain no large particles or coarse fiber. Theseadditives, coloring agents, and fluorescent agents may be incorporatedinto the thermoplastic resin through kneading. Alternatively, theseadditives, coloring agents, and fluorescent agents may be added to thethermoplastic resin during a shape-imparting process.

The organic polymer composition containing silica-coated zinc oxidepowder and a thermoplastic resin can be obtained by mixing the coatedzinc oxide powder and the thermoplastic resin. However, in addition tosimply mixing the coated zinc oxide powder and the thermoplastic resin,kneading of the mixture is preferably performed in order to enhanceuniformity, because the coated zinc oxide powder has a small particlesize. The additives, coloring agents, fluorescent agents, and similaragents may be added to the composition during mixing or kneading.

Mixing of the silica-coated zinc oxide powder and the thermoplasticresin can be performed by use of a mixer such as a V-shape mixer or aHenschel mixer. Kneading can be performed by use of a batch kneader suchas a Banbury mixer; or a continuous kneader such as a single extruder, atwin extruder, or a continuous mixer.

The organic polymer composition containing silica-coated zinc oxidepowder and a thermoplastic resin may be used singly or may be added as amaster batch to a thermoplastic resin for dilution.

Examples of the thermoplastic resin for dilution include, but are notlimited to, polyethylene, polypropylene, polystyrene, polyethyleneterephthalate, AS resins, ABS resins, AES resins, polyvinylidenechloride, methacrylic resin, polyvinyl chloride, polyamides,polycarbonates, polyallyl esters, polyimides, polyacetals, polyetherketones, polyether sulfones, polyphenyl oxides, and polyphenylenesulfides.

The organic polymer composition of the present invention may be usedsingly or as a master batch. Such polymer compositions can be subjectedto molding methods generally applied to thermoplastic resins, such asinjection molding, blow molding, extrusion molding, calender molding,flow molding, compression molding, melt-blown molding, and the spun bondmethod, whereby shape-imparted products such as fiber, thread, film,sheets, tapes, and injection-molded products and shaped bodies such ashollow thread, pipes, and bottles can be produced. Alternatively, thecomposition can be subjected to secondary molding methods generallyapplied to thermoplastic resins such as vacuum forming, air pressureforming, and laminate molding.

No particular limitation is imposed on the shape of the shape-impartedproducts such as fiber, thread, film, sheets, tapes, andinjection-molded products and shaped bodies such as hollow thread,pipes, and bottles formed from the organic polymer composition of thepresent invention, and any shape-imparted products; i.e., thin (or fine)to thick products, can be produced. However, a characteristic feature ofthe organic polymer composition of the present invention lies in thatthin or fine shape-imparted products, which are difficult to producethrough a customary method, can be produced. Thus, the composition issuitable for production of fine fiber or thread, thin film or tapes,etc.

These shape-imparted products may have single-layer structure ormulti-layer structures. When a shape-imparted product has a multi-layerstructure, a UV shielding ability can be enhanced by providing a toplayer of the shape-imparted product from the organic polymer compositionof the present invention.

Processability of the organic polymer composition of the presentinvention itself or a composition containing the organic polymercomposition as a master batch, when subjected to a variety ofshape-imparting processes, can be evaluated by use, as an index, of theprocessability thereof attained with a small extruder. Specifically,when a predetermined amount of each organic polymer composition masterbatch is extruded, by use of a Labo Plastomill (product of Toyo SeikiSeisaku-sho), along with a counterpart resin, and the increase inpressure of kneaded resin with respect to the initial kneaded resinpressure is measured, such an increase in pressure serves as an indexfor processability not only upon use of a Labo Plastomill, but alsoprocessability in several other cases; for example, the case in whichthin film is formed through inflation film molding by use of an organicpolymer composition master batch itself or the master batch diluted withanother thermoplastic resin, or the case in which fiber is formedthrough a multi-filament forming process. When the increase in pressureof kneaded resin induced by the organic polymer composition master batchis small, processability of the organic polymer composition master batchupon use of a Labo Plastomill is evaluated as being of an excellentlevel. In addition, puncturing, or a similar phenomenon, is prevented information of thin film, and breakage of thread or a similar phenomenonis prevented in formation of fine fibers or a similar material, wherebyexcellent processability can be attained.

For example, when the organic polymer composition master batchcontaining polypropylene as a thermoplastic resin is extruded with acounter resin, a full-flight-type 20 mmφ extruder having screens of 100,630, 100, 80, and 60 meshes attached thereto is operated at a rotationalspeed of 45 rpm, and under the following temperature conditions: 230(inlet)-230-230-230° C. The increase in resin pressure as measured afterextrusion of 3 kg of the master batch, relative to the resin pressureimmediately after start of kneading, is preferably 5 MPa or less, morepreferably 3 MPa or less.

For example, when the organic polymer composition master batchcontaining polyamide as a thermoplastic resin is extruded with a counterresin, a full-flight-type 20 mmφ extruder having screens of 100, 630,100, 80, and 60 meshes attached thereto is operated at a rotationalspeed of 45 rpm, and under the following temperature conditions: 270(inlet)-270-270-270° C. The increase in resin pressure as measured afterextrusion of 3 kg of the master batch, relative to the resin pressureimmediately after start of kneading, is preferably 10 MPa or less, morepreferably 5 MPa or less.

Shape-imparted products such as fiber, thread, film, sheets, tapes, andinjection-molded products and shaped bodies such as hollow thread,pipes, and bottles, which are formed from the organic polymercomposition of the present invention, inter alia, fiber, film, sheets,and tapes, can be used singly or shape-imparted into a multi-layerstructure including a substrate and a surface layer formed of theorganic polymer composition, through co-extrusion with anotherthermoplastic resin, molding with a substrate, or attaching to a surfaceof a substrate. No particular limitation is imposed on the film, sheet,or tape thickness, and the thickness is appropriately selected inaccordance with use thereof. Generally, the thickness is 0.0005 to 5.0mm, preferably 0.001 to 1.0 mm, more preferably 0.001 to 0.1 mm. Noparticular limitation is imposed on the thickness of thread, and thethickness is appropriately selected in accordance with the use thereof.Generally, the thickness is 1 to 500 deniers, preferably 1 to 100deniers, more preferably 1 to 50 deniers.

Shape-imparted products such as fiber, thread, film, sheets, tapes, andinjection-molded products and shaped bodies such as hollow thread,pipes, and bottles, which are formed from the organic polymercomposition of the present invention, inter alia, fiber, film, sheets,and tapes, can be attached to a surface of a substrate by the mediationof an adhesive. Examples of the adhesive which can be employed includeurethane-based, acryl-based, poly(vinyl alcohol)-based, andvinyl-acetate-based adhesives. Alternatively, a peelable protective filmmay also be provided, by the mediation of an adhesion layer, onshape-imparted products such as fiber, thread, film, sheets, tapes, andinjection-molded products and shaped bodies such as hollow thread,pipes, and bottles, formed from the organic polymer composition of thepresent invention, inter alia, fiber, film, sheets, and tapes. Examplesof the protective film which can be employed include coated paper havinga silicone resin coating as a releasing layer, and biaxially stretchedpoly(ethylene terephthalate) film. The thus-provided structure having anadhesion layer and a protective film can be attached to any substratesurface by peeling the protective film.

Shape-imparted products such as fiber, thread, film, sheets, tapes, andinjection-molded products and shaped bodies such as hollow thread,pipes, and bottles, which are formed from the organic polymercomposition of the present invention, inter alia, fiber, film, sheets,and tapes, can be printed with a picture or embossed. Three-dimensionalstructures can be formed from these products.

No particular limitation is imposed on the material and shape of thesubstrate, so long as the substrate allows formation of a UV-shieldinglayer on its surface. Examples of the material for forming the substrateinclude metals such as iron, aluminum, and copper; ceramics such asglass and porcelain; inorganic materials such as gypsum, calciumsilicate, and cement; plastics such as polyvinyl chloride, polyester,polyolefin, polycarbonate, polyamide, acrylic resin, ABS resin,polystyrene, phenolic resin, and FRP; organic materials such as wood,plywood, and paper; and fibers such as glass fiber, carbon fiber, andpolyester fiber. Multi-layer structures can be formed fromshape-imparted products such as fiber, thread, film, sheets, tapes, andinjection-molded products and shaped bodies such as hollow thread,pipes, and bottles, formed from the organic polymer composition of thepresent invention. No particular limitation is imposed on the shape anddimensions of the substrate, and the substrate may have arbitrary shapessuch as film, sheet, fiber, woven fabric, non-woven fabric, orthree-dimensional structure.

The shape-imparted products or multi-layer structures as described abovemay be used singly or may be attached to a portion of another structure.No particular limitation is imposed on the additional structure.Examples include inorganic structures such as metal, concrete, glass,and ceramic structures; organic structures such as paper, plastic, wood,and leather structures; and combinations thereof. Examples of specificproducts include packaging materials, building materials, machinery,vehicles, glass products, electric appliances, agricultural materials,electronic apparatus, tools, tableware, bath products, toiletryproducts, furniture, clothing, cloth products, fibers, leather products,paper products, sporting goods, futon, containers, eyeglasses,signboards, piping, wiring, brackets, sanitary materials, automobileparts, tents, stockings, socks, gloves, and masks.

EXAMPLES

The present invention will be described in further detail with referenceto Examples and Comparative Examples; however, the present invention isnot limited to the Examples.

Examples and Comparative Examples, which will be described later, wereevaluated for the following items.

(Measurement of Silica Film Thickness)

Silica-coated zinc oxide fine particles were observed under atransmission electron microscope (JEM 2010, product of JEOL;acceleration voltage: 200 V); and the thickness of silica coating onparticle surfaces (a film portion observed to cover substrate particlesand having low contrast) was measured.

(Average Primary Particle Size)

Silica-coated zinc oxide fine particles were observed under atransmission electron microscope (JEM 2010, product of JEOL;acceleration voltage: 200 V); 100 arbitrary particles were selected; andparticle sizes of the selected particles were measured, from which theiraverage particle size was calculated.

(IR Spectrum Measurement)

A transmission infrared absorption spectrum (by FT-IR-8000 of JASCO) ofthe silica film of silica-coated zinc oxide fine particles was measuredby the KBr method (ratio of light-emitting particles to powder of KBrwas 1:3 (by mass)). Transmittances and absorption-peak absorbances at1150 to 1250 cm⁻¹ and 1000 to 1100 cm⁻¹ were calculated; and absorptionpeak intensity ratio I (I=I1/I2; wherein I1 denotes absorption-peakabsorbance at 1150 to 1250 cm⁻¹ and I2 denotes absorption-peakabsorbance at 1000 to 1100 cm⁻¹) was obtained.

(Measurement of Refractive Index)

Refractive index of silica film, which was formed on a silicon waferdipped into the system in order to synthesize silica-coated zinc oxideparticles, was measured by use of an ellipsometer (LASSER ELLIPSOMETERESM-LA, product of ULVAC).

(Tetralin Auto-oxidation Method)

This method is described in “Titanium Oxide—Physical Properties andApplication Techniques,” Manabu KIYONO, GIHODO SHUPPAN, p. 196-197,1991. Measurement conditions were such that temperature: 40° C.,tetralin: 20 mL, and zinc oxide: 0.02 g.

(Measurement of Dye Color Fading Rate: Sunset Yellow Method)

The thus-obtained silica-coated zinc oxide particles, uncoated zincoxide particles (material zinc oxide particles), and commerciallyavailable zinc oxide (ZnO 350, product of Sumitomo Osaka Cement), whichserved as test materials, were measured for dye color fading rate by thesunset yellow method.

First, sunset yellow FCF (dye, product of Wako Pure Chemical Industries)was dissolved in glycerin (98 mass %) to attain a dye concentration of0.02 mass %. Each test material was dispersed in an amount of 0.067 mass%, and the test-material dispersed solution was irradiated with UV rays(UV ray intensity: 1.65 mW/cm²). With an optical path length set to 1mm, absorbance at 490 nm, which is a maximum absorption wavelength ofsunset yellow FCF, was measured continuously by use of a spectralphotometer (UV-160, product of SHIMADZU). Subsequently, a difference(ΔABS₄₉₀/hour) between the measured absorbance decreasing rate and thatof a control test solution (not containing zinc oxide) was calculated.

(Measurement of Organic UV Absorber Decomposition Rate: Parasol 1789Method)

The thus-obtained silica-coated zinc oxide particles, uncoated zincoxide particles (material zinc oxide particles), and commerciallyavailable zinc oxide (ZnO 350, product of Sumitomo Osaka Cement), whichserved as test materials, were measured for their rates of decompositionof Parasol 1789, which is an organic UV absorber.

Specifically, each test material was dispersed into a polyethyleneglycol 300 solution containing 0.045 mass % of4-tert-butyl-4′-methoxybenzoylmethane (Parasol 1789) so as to obtain aslurry containing the test material in an amount of 1 mass %. The slurry(1.2 g) was then placed in a glass container, which was irradiated withUV rays for 10 hours (1.65 mW/cm²). Subsequently, 1 g of the slurry wassampled, and isopropyl alcohol (2 mL), hexane (2 mL), and distilledwater (3 mL) were successively added. Through stirring, Parasol 1789 wasextracted to the hexane phase; and absorbance of the hexane phase(optical path length: 1 mm, wavelength: 340 nm) was measured by use of aspectral photometer (UV-160, SHIMADZU). Subsequently, a difference(ΔABS₃₄₀/hour) between the measured absorbance decreasing rate at 340 nmand that of a control test solution (not containing zinc oxide) wascalculated.

(Measurement of Percent Organic UV Absorber Decomposition: Ethylhexylp-Methoxycinnamate Method)

The thus-obtained silica-coated zinc oxide fine particles, uncoated zincoxide particles (material zinc oxide particles), and commerciallyavailable zinc oxide (ZnO 350, product of Sumitomo Osaka Cement), whichserved as test materials, were measured for their percent decompositionof ethylhexyl p-methoxycinnamate, which is an organic UV absorber.

Specifically, each test material was dispersed into a polyethyleneglycol 300 solution containing 0.05 mass % of 2-ethylhexylp-methoxycinnamate so as to obtain a slurry containing the test materialin an amount of 0.33 mass %. The slurry (1.2 g) was then placed in aglass container, which was irradiated with UV rays for 90 minutes (1.65mW/cm²). Subsequently, 1 g of the slurry was sampled, and isopropylalcohol (2 mL), hexane (2 mL), and distilled water (3 mL) weresuccessively added. Through stirring, 2-ethylhexyl p-methoxycinnamatewas extracted to the hexane phase; and absorbance of the hexane phase(optical path length: 1 mm, wavelength: 300 nm) was measured by use of aspectral photometer (UV-160, product of SHOMADZU). Subsequently, thepercent decomposition of 2-ethylhexyl p-methoxycinnamate was obtainedfrom the difference between the measured absorbance decreasing rate at300 nm and that of a control test solution (not containing zinc oxide).

(Zinc Releasability Test)

Release of zinc ions from silica-coated zinc oxide containing powder towater was evaluated as follows.

Each of silica-coated zinc oxide containing powder (includingsilica-coated zinc oxide containing powder having hydrophobic surface)and uncoated zinc oxide powder was dispersed into solutions of variouspHs in an amount of 5 mass %, followed by stirring at 25° C. for 3hours. Subsequently, each of the powder-dispersed solutions wassubjected to centrifugal separation for precipitation; and the quantityof zinc ions within the supernatant was measured by use of an atomicabsorption spectrophotometer (Z-8200, product of Hitachi). (Amount ofLarge particles of 5 μm or More)

When a sample is powder containing silica-coated zinc oxide particlesobtained by coating the surfaces of zinc oxide particles with silica, 20g of the sample is accurately weighed, and is then placed in purifiedwater (1800 mL) at room temperature, followed by thorough stirring. Anappropriate amount (10 mL) of a dispersant, such as an 10% aqueoussolution of sodium hexametaphosphate is added to the sample-added water,and the resultant solution is then stirred, and subjected to ultrasonicdispersion for 10 minutes. The ultrasonic dispersion can be performed byuse of an ultrasonic homogenizer (US-300T, product of Nippon SeikiSeisakusho, output: 300 W, oscillation frequency : 20 kHz).Subsequently, the slurry is poured onto a precision micro-mesh sievehaving a mesh-size of 5 μm set on a particle classifier of Yokohama Rika(model PS-80). Subsequently, wet precision classification is performedby means of an ultrasonic vibrator, an electromagnetic vibrator, and asuction pump, which are incorporated in the apparatus. After completionof the classification, water in a washing bottle is jetted to the sievein order to collect the powder remaining on the sieve, which powder isthen placed in a glass container together with purified water. The glasscontainer is placed in a 110° C. drying apparatus so as to evaporate thewater. The remaining residue is collected and weighed. The weight ratioof the residue to the original sample (20 g) represents the amount oflarge particles having a particle size of 5 μm or greater.

When a sample is powder containing silica-coated zinc oxide particlesobtained by coating the surfaces of zinc oxide particles with silica andthen surface-treating the particles by use of a hydrophobicity-impartingagent, in the above-described classification operation, wet precisionclassification can be performed by using, instead of water, anequi-volume mixture solvent of water and methanol as the solvent otherthan the dispersant. After completion of the classification, the powderremaining on the sieve was dried in air, and is then placed in a 110° C.drying apparatus. The remaining residue is collected and weighed. Theweight ratio of the residue to the original sample (20 g) represents theamount of large particles having a particle size of 5 μm or greater.

(Kneaded Resin Pressure)

A variety of organic polymer composition master batches each containingsilica-coated zinc oxide containing powder and a thermoplastic resinwere each measured for kneaded resin pressure by use of a LaboPlastomill of Toyo Seiki Seisaku-sho, and they were evaluated for theirprocessability. Kneading conditions of the Labo Plastomill were asfollows. A full-flight-type 20 mmφ extruder having screens of 100, 630,100, 80, and 60 meshes attached thereto was operated at a rotationalspeed of 45 rpm, and temperature conditions were determined inaccordance with the type of resin. The processability of each organicpolymer composition master batch was evaluated on the basis of a rise inresin pressure as measured after extrusion of 3 kg of the master batch,relative to the resin pressure immediately after start of kneading.

(Suppression of Impairment of Weather Resistance Attributable toPhotocatalytic Action)

An organic polymer composition master batch which contains 20%silica-coated zinc oxide containing powder was added to a dilution resinin such a manner that the amount of the silica-coated zinc oxidecontaining powder becomes 1%. From the thus-obtained resin, film havinga thickness of 100 μm was obtained by use of a 25 mm T-die film moldingmachine of Chuo Kikai Seisakusho.

The thus-obtained film was placed in a Sunshine Super-Long-Life WeatherMeter WEL-SUN-HCH of Suga Test Instruments for 180 hours in order totest impairment of weather resistance attributable to photocatalyticaction.

Evaluation for impairment of weather resistance attributable tophotocatalytic action was performed on the basis of a change in haze ofthe film. Specifically, before and after being placed in the SunshineSuper-Long-Life Weather Meter, haze of the film was measured by use of areflecto/transmissometer HR-100 of Murakami Color Research Laboratory,and the impairment of weather resistance attributable to photocatalyticaction was evaluated on the basis of a change in the measured haze. Itcan be judged that the smaller the change in haze, the more theimpairment of weather resistance, attributable to photocatalytic action,is suppressed.

Example 1

In a reactor (50 L), deionized water (18.25 L), ethanol (22.8 L, productof Junsei Chemical Co., Ltd.), and 25 mass % aqueous ammonia (124 mL,product of Taisei Chemical Industries Co., Ltd.) were mixed, and zincoxide particles serving as raw material (1.74 kg, high-purity zinc oxideUFZ-40; primary particle size 27 nm, product of Showa Titanium Co.,Ltd.) were dispersed in the mixture, to thereby prepare a suspension A.Subsequently, tetraethoxysilane (1.62 L, product of GE ToshibaSilicones) and ethanol (1.26 L) were mixed, to thereby prepare asolution B.

While suspension A was stirred, solution B was added over nine hours ata constant speed, and the resultant solution was allowed to ripen for 12hours. Film formation and ripening were conducted at 45° C. Thereafter,solid matter was separated through centrifugal filtration, dried invacuum for 12 hours at 50° C., and dried by the application of 80° C.air for 12 hours. Subsequently, the dried solid matter was milled by ajet mill, whereby silica-coated zinc oxide fine particles were obtained.

Through the KBr method, transmission infrared absorption spectra of thethus-obtained silica-coated zinc oxide were determined. As a result, at1,000 to 1,200 cm⁻¹, the thus-obtained silica-coated zinc oxideexhibited absorption attributed to Si—O—Si stretching vibrations, and at2,800 to 3,000 cm⁻¹, the silica-coated zinc oxide did not exhibitabsorption attributed to C—H stretching vibrations, demonstrating thatthe produced film was silica.

Moreover, the following were measured: primary particle size, silicafilm thickness, ratio I of infrared absorption intensity as measured ininfrared absorption spectra, refractive index of silica film,photocatalytic activity measured by the tetralin auto-oxidation method,etc. The results are shown in Table 1.

The dye color fading rate of the silica-coated zinc oxide fine particlesmeasured by the tetralin auto-oxidation method was found to be 0.1(ΔABS₄₉₀/hr) or less, proving that suppression of decomposition of thedye was of low level as compared with uncoated product or commerciallyavailable zinc oxide.

The decomposition rate of organic UV absorber of the silica-coated zincoxide fine particles measured by the Parasol 1789 method was found to be0.02 (ΔABS₃₄₀/hr) or less, proving that the particles have considerablylow decomposition ability of the organic UV absorber as compared withuncoated product or commercially available zinc oxide.

The decomposition ratio of the silica-coated zinc oxide fine particleswas found to be 5% or less, proving that the particles have lowdecomposition ability of the organic UV absorber as compared withuncoated product or commercially available zinc oxide. TABLE 1Measurement Measurement items values Primary particle size (nm) 27Silica film thickness (nm) 3 Infrared absorption peak intensity ratio0.45 I value Refractive index of silica film 1.443 Tetralinauto-oxidation activity (Pa/min) 39 Sunset yellow color fading rate(ΔABS₄₉₀/hr) 0 Decomposition rate as determined by the 0.002 Parasol1789 method (ΔABS₃₄₀/hr) Decomposition ratio as determined by the 0.5ethylhexyl paramethoxycinnamate method (%)

To silica-coated zinc oxide (97 parts by mass), a solution ofdimethylpolysiloxane (3 parts by mass, KF96-100 CS, product of Shin-EtsuChemical Co., Ltd.) in dichloromethane was added, and the mixture wasblended well by a Henschel mixer (product of Mitsui-Miike).Subsequently, solvent was removed at 90° C. under drying in vacuum, andthe dried material was fired for six hours at 200° C., to thereby yieldsilica-coated zinc oxide having a hydrophobicized surface. The amount oflarge particles (5 μm or more) contained in the thus-obtainedsilica-coated zinc oxide containing powder, was found to be 1.6 mass %.Results regarding the release of zinc ions were shown in Table 2.

The silica-coated zinc oxide of the present invention exhibited a veryreduced release of zinc ions at different pH values as compared withuncoated zinc oxide, and the amount of zinc ions in purified water wasfound to be as low as 0.5 ppm or less. When the surface of thesilica-coated zinc oxide of the present invention was furtherhydrophobicized, release of zinc ions at different pH values wassuppressed. Therefore, organic polymer compositions or shape-impartedproducts thereof are also expected to exhibit the excellent effect ofpreventing zinc ion release, which release may occur upon contact withan acid or alkaline solution. TABLE 2 Released zinc ion (ppm) 0.01%Nitric Purified 1% NH₃ acid water (pH solution solution 6.4) (pH 11.4)(pH 2.5) Silica-coated zinc oxide <0.5 20 9 particles of Example 1Surface-hydrophobicized <0.5 2 1 silica-coated zinc oxide particles ofExample 1 Uncoated starting zinc 8 480 91 oxide particles of Example 1

Moreover, the surface-hydrophobicized powder was subjected to dry-formatprecision classification by use of a turbo classifier (product ofNisshin Engineering K.K.). The amount of large particles (5 μm or more)in the thus-obtained silica-coated zinc oxide containing powder wasfound to be 0.02 mass %.

The thus-obtained powder containing silica-coated zinc oxide (20 partsby mass) and polypropylene (80 parts by mass, PW600N, product ofSunAllomer Ltd.) were mixed in a super-mixer (product of Kawada K.K.)for three minutes at 600 rpm. Subsequently, the mixture was kneaded in a30 mm different-direction double screw extruder (product of NakataniK.K.), to thereby yield an organic polymer composition master batch.

Processability of the thus-obtained organic polymer composition masterbatch by a Labo Plastomill was evaluated under thermal condition of 230(inlet)-230-230-230° C. As is apparent from Table 3, rise in resinpressure was found to be as low as 1.3 MPa, and processability was good.

The thus-obtained organic polymer composition master batch was dilutedwith PW60ON (product of SunAllomer Ltd.), and a test was conducted toinvestigate the level at which weather resistance was impaired byphotocatalytic action.

The hazes of the film before and after the film was subjected to aSunshine Super-long-Life Weather Meter were 18.2 and 18.6, respectively,and the variation of the hazes was found to be as small as 0.4,indicating minimized impairment of weather resistance caused byphotocatalytic action. This shows that the thus-obtained silica-coatedzinc oxide fine particles cause extremely low impairment of weatherresistance by photocatalytic action to polypropylene.

Example 2

The procedure of Example 1 was repeated, except that polyethylene (JapanPolyolefins Co., Ltd., JH607C) was used instead of polypropylene(product of SunAllomer Ltd., PW600N), to thereby yield an organicpolymer composition master batch.

In a manner similar to that described in Example 1, the processabilityof the thus-obtained organic polymer composition master batch by a LaboPlastomill was evaluated. As a result, the rise in resin pressure wasfound to be as low as 0.7 MPa, and processability was good.

The thus-obtained organic polymer composition master batch was dilutedwith polyethylene (product of Japan Polyolefins Co., Ltd., JH607C), anda test on impaired weather resistance caused by photocatalystic actionwas performed. As a result, the haze variation was found to be as smallas 0.2, indicating minimized impairment of weather resistance caused byphotocatalytic action. This shows that the thus-obtained silica-coatedzinc oxide fine particles cause extremely low impairment of weatherresistance caused by photocatalytic action to polyethylene.

Example 3

The procedure of Example 1 was repeated, except that polyamide (productof EMS-Showa Denko K.K., A28GM) was used instead of polypropylene(product of SunAllomer Ltd., PW600N), to thereby yield an organicpolymer composition master batch.

The procedure of Example 1 was repeated, except that the thermalcondition was changed to 270 (inlet)-270-270-270° C., and theprocessability of the thus-obtained organic polymer composition masterbatch, by a Labo Plastomill, was evaluated. As a result, the rise inresin pressure was found to be as low as 2.1 MPa, and processability wasgood.

Example 4

The procedure for producing the silica-coated zinc oxide fine particlesof Example 1 was repeated, except that titanium oxide powder(high-purity titanium oxide F-4; primary particle size 30 nm, product ofShowa Titanium Co., Ltd.) was used as raw material, to obtainsilica-coated titanium oxide fine particles.

To the thus-obtained silica-coated titanium oxide (94 parts by mass), asolution of dimethylpolysiloxane (6 parts by mass, KF96-100CS, productof Shin-Etsu Chemical Co., Ltd.) in dichloromethane was added, and themixture was blended well by a Henschel mixer (product of Mitsui-Miike).Subsequently, the solvent was removed at 90° C. by drying under vacuum,and the dried material was fired for six hours at 200° C., to therebyyield silica-coated titanium oxide having hydrophobicized surface. Theamount of large particles (5 μm or more) contained in the thus-obtainedsilica-coated titanium oxide powder, was found to be 1.5% by mass.

Moreover, the surface-hydrophobicized powder was subjected to dry-formatprecision classification by use of a turbo classifier (product of NissinEngineering K.K.). The amount of large particles (5 μm or more) in thethus-obtained silica-coated titanium oxide powder was found to be 0.02mass %.

Subsequently, thus-classified silica-coated titanium oxide powder (30parts by mass) and the classified (as Example 1) silica-coated zincoxide powder (70 parts by mass) were mixed uniformly, and then a powdercomprising silica-coated zinc oxide fine particles was obtained.

The amount of large particles (5 μm or more) in the thus-obtained powdercomprising silica-coated zinc oxide fine particles was found to be 0.02mass %.

The powder comprising silica-coated zinc oxide fine particles (20 partsby mass) and polypropylene (80 parts by mass, PW600N, product ofSunAllomer Ltd.) were mixed in a super-mixer (product of Kawada K.K.)for three minutes at 600 rpm. Subsequently, the mixture was kneaded in a30 nm different-direction double screw extruder (product of NakataniK.K.), to thereby yield an organic polymer composition master batch.

In a manner similar to that described in Example 1, processability ofthe thus-obtained organic polymer composition master batch by a LaboPlastomill was evaluated. As a result, rise in resin pressure was foundto be as low as 1.0 Mpa, and processability was good.

On the other hand, when the thus-obtained organic polymer compositionmaster batch was subjected to a test in a manner similar to thatdescribed in Example 1 on weather resistance impairment caused byphotocatalytic action, the haze variation was found to be as low as 0.7,indicating minimal impairment of weather resistance caused byphotocatalytic action.

Comparative Example 1

The procedure of Example 1 was repeated, except that unclassified powdercontaining silica-coated zinc oxide was used instead of classifiedpowder containing silica-coated zinc oxide, to thereby yield an organicpolymer composition master batch.

In a manner similar to that described in Example 1, processability ofthe thus-obtained organic polymer composition master batch by a LaboPlastomill was evaluated. As a result, rise in resin pressure was foundto be as high as 8.6 MPa and the processability was poor.

However, as a result of a test conducted similar to that described inExample 1 on the thus-obtained organic polymer composition master batchfor investigating the level at which weather resistance was impaired byphotocatalystic action, the haze variation of the film was found to beas small as 0.5, indicating minimized impairment of weather resistancecaused by photocatalytic action. This shows that the thus-obtainedsilica-coated zinc oxide fine particles undergo minimal impairment ofweather resistance caused by photocatalytic action againstpolyplopylene.

Comparative Example 2

The procedure of Example 2 was repeated, except that the powdercontaining silica-coated zinc oxide used in Comparative Example 1 wasemployed, to thereby yield an organic polymer composition master batch.In a manner similar to that described in Example 2, the processabilityof the thus-obtained organic polymer composition master batch by a LaboPlastomill was evaluated. As a result, a rise in resin pressure wasfound to be as high as 3.5 MPa, and the processability was poor.

On the other hand, when the thus-obtained organic polymer compositionmaster batch was subjected to a test in a manner similar to thatdescribed in Example 2 on weather resistance impairment caused byphotocatalytic action, the haze variation of the film was found to be aslow as 0.3, indicating minimal impairment of weather resistance causedby photocatalytic action. This shows that the thus-obtainedsilica-coated zinc oxide fine particles undergo minimal impairment ofweather resistance caused by photocatalytic action against polyethylene.

Comparative Example 3

The procedure of Example 3 was repeated, except that the powdercontaining silica-coated zinc oxide used in Comparative Example 1 wasemployed, to thereby yield an organic polymer composition master batch.In a manner similar to that described in Example 3, processability ofthe thus-obtained organic polymer composition master batch by a LaboPlastomill was evaluated. As a result, rise in resin pressure was foundto be as high as 15 MPa or higher, and the processability was poor.

Comparative Example 4

The procedure of Example 1 was repeated, except that high-purity zincoxide (UFZ-40; primary particle size 27 nm, product of Showa TitaniumCo., Ltd.) was used instead of the powder containing silica-coated zincoxide particles before being hydrophobicized, to thereby yield anorganic polymer composition master batch. In a manner similar to thatdescribed in Example 1, processability of the thus-obtained organicpolymer composition master batch by a Labo Plastomill was evaluated. Asa result, the rise in resin pressure was found to be as high as 4.2 MPa,and processability was poor.

In a manner similar to that described in Example 1, the thus-obtainedorganic polymer composition master batch was subjected to a test onweather resistance impairment caused by photocatalystic action. As aresult, the haze variation of the film was found to be as high as 8.2,indicating significant impairment of weather resistance caused byphotocatalytic action. This shows that the thus-obtained silica-coatedzinc oxide fine particles undergo significant impairment of weatherresistance caused by photocatalytic action against polypropylene.

Comparative Example 5

The procedure of Example 2 was repeated, except that high-purity zincoxide (UFZ-40; primary particle size 27 nm, product of Showa TitaniumCo., Ltd.) was used instead of the powder containing silica-coated zincoxide particles before being hydrophobicized, to thereby yield anorganic polymer composition master batch. In a manner similar to thatdescribed in Example 2, processability of the thus-obtained organicpolymer composition master batch by a Labo Plastomill was evaluated. Asa result, the rise in resin pressure was found to be as high as 6.5 MPa,and the processability was poor.

In a manner similar to that described in Example 2, the thus-obtainedorganic polymer composition master batch was subjected to a test ofweather resistance impairment caused by photocatalystic action. As aresult, the haze variation of the film was found to be as high as 6.5,indicating significant impairment of weather resistance caused byphotocatalytic action. This shows that the thus-obtained silica-coatedzinc oxide fine particles undergo significant impairment of weatherresistance caused by photocatalytic action against polyethylene.

Comparative Example 6

An aqueous suspension of zinc oxide particles (high-purity zinc oxideUFZ-40; primary particle size 27 nm, product of Showa Titanium Co.,Ltd.) (ZnO concentration: 50 g/L) was heated to 80° C. Under stirring,an aqueous solution of sodium silicate (the ratio of SiO₂ to zinc oxide:10% by weight) was added to the suspension. The mixture was allowed toripen for 10 minutes and, subsequently, sulfuric acid was added over 60minutes under stirring, whereby the mixture was neutralized to pH 6.5.The mixture was allowed to ripen for 30 minutes, and then the resultantsuspension was subjected to filtration and washing with water, followedby drying with heat for five hours at 130° C. The thus-obtained driedproduct was milled with a jet mill, to thereby yield powder containingsilica-coated zinc oxide. The procedure of Example 1 was repeated,except that the powder was used instead of unhydrophobicizedsilica-coated zinc oxide fine particles, whereby an organic polymercomposition master batch was obtained.

In a manner similar to that described in Example 1, processability ofthe thus-obtained organic polymer composition master batch by a LaboPlastomill was evaluated. As a result, rise in resin pressure was foundto be as low as 2.2 MPa, and processability was good.

In a manner similar to that described in Example 1, the thus-obtainedorganic polymer composition master batch was subjected to a test ofweather resistance impairment caused by photocatalystic action. As aresult, the haze variation of the film was found to be as high as 3.5,indicating significant impairment of weather resistance caused byphotocatalytic action. This shows that the thus-obtained silica-coatedzinc oxide fine particles undergo significant impairment of weatherresistance caused by photocatalytic action against polypropylene. TABLE3 Deterioration of weatherability by photocatalyst action ProcessabilityHaze of film Rise in After 180 kneading resin hours of Classifi-pressure of weather cation Resin composition Initial meter testVariation Ex. 1 Yes PP 1.3 MPa 18.2 18.6 0.4 Ex. 2 Yes PE 0.7 MPa 13.013.2 0.2 Ex. 3 Yes PA 2.1 MPa — — — Ex. 4 Yes PP 1.0 MPa 19.3 20.0 0.7Comp. No PP 8.6 MPa 18.5 19.0 0.5 Ex. 1 Comp. No PE 3.5 MPa 13.1 13.40.3 Ex. 2 Comp. No PA  15 MPa — — — Ex. 3 Comp. Yes PP 4.2 MPa 18.6 26.88.2 Ex. 4 Comp. Yes PE 3.1 MPa 12.6 19.1 6.5 Ex. 5 Comp. Yes PP 2.8 MPa17.5 21.0 3.5 Ex. 6

INDUSTRIAL APPLICABILITY

The present invention facilitates processing of thin films and thinfibers endowed with sufficient UV shielding ability without invitingimpaired weather resistance which may otherwise be caused byphotocatalytic action. The invention provides silica-coated zinc oxidepowder containing large particles of 5 μm or more in an amount of 0.1mass % or less; organic polymer compositions containing the powder; andshape-imparted products produced from the compositions.

1. A powder comprising silica-coated zinc oxide fine particles in whichthe surface of each particle is coated with silica, wherein largeparticles of 5 μm or more account for 0.1 mass % or less and this amountis obtained by a dry-format classification.
 2. A powder comprisingsurface-hydrophobicized silica-coated zinc oxide fine particles in whichthe silica-coated zinc oxide fine particles whose surfaces have beencoated with silica are further treated with a hydrophobicity-impartingagent, wherein large particles of 5 μm or more account for 0.1 mass % orless and this amount is obtained by a dry-format classification.
 3. Thepowder as claimed in claim 2, wherein the hydrophobicity-imparting agentis one or more members selected from the group consisting of siliconeoils, alkoxysilanes, silane coupling agents, and higher fatty acidsalts.
 4. The powder as claimed in any of claims 1 through 3, whereinthe silica-coated zinc oxide fine particles have silica coating of 0.5to 100 nm in thickness.
 5. The powder as claimed in claim 1 or 2,wherein the silica-coated zinc oxide fine particles have an averageprimary particle size of 1 to 200 nm.
 6. The powder as claimed in claim2, wherein the surface-hydrophobicized, silica-coated zinc oxide fineparticles have an average primary particle size of 5 to 120 nm and asilica-film thickness of 0.5 to 25 nm.
 7. The powder as claimed in claim1 or 2, wherein the ratio I of infrared absorption peak intensity ofsilica film of the silica-coated zinc oxide fine particles at 1150 to1250 cm⁻¹ to that at 1000 to 1100 cm⁻¹ as determined on an infraredabsorption spectrum is 0.2 or more (I=I1/I2; wherein I1 denotesabsorption peak intensity at 1150 to 1250 cm⁻¹ and I2 denotes absorptionpeak intensity at 1000 to 1100 cm⁻¹), and the silica film has arefractive index of 1.435 or more.
 8. The powder as claimed in claim 1or 2, wherein the powder exhibits a photocatalytic activity of 60 Pa/minor less as measured through the tetralin auto-oxidation method.
 9. Thepowder as claimed in claim 1 or 2, wherein the powder exhibits a dyecolor fading rate (ΔABS₄₉₀/hour) of 0.1 or less as measured through thesunset yellow method.
 10. The powder as claimed in claim 1 or 2, whereinthe powder exhibits an organic UV absorber decomposition rate(ΔABS₃₄₀/hour) of 0.01 or less as measured through the Parasol method.11. The powder as claimed in claim 1 or 2, wherein the powder exhibits apercent organic UV absorber decomposition of 5% or less as measuredthrough the ethylhexyl p-methoxycinnamate method.
 12. The powdercomprising silica-coated zinc oxide fine particles as claimed in claim 1or 2, which contains titanium oxide.
 13. The powder comprisingsilica-coated zinc oxide fine particles as claimed in claim 12, whereintitanium oxide in an amount of 2 parts by mass to 5 parts by mass isfurther contained based on zinc oxide of 10 parts by mass.
 14. Thepowder comprising silica-coated zinc oxide fine particles as claimed inclaim 12, wherein at least one part of titanium oxide is coated withsilica.
 15. The powder comprising silica-coated zinc oxide fineparticles as claimed in claim 12, wherein the titanium oxide contains amixed crystal having a titanium-oxygen-silicon bond in its primaryparticles.
 16. The powder comprising silica-coated zinc oxide fineparticles as claimed in claim 15, wherein when the BET specific surfacearea of titanium oxide is represented by “A m²/g” and the SiO₂ contentis represented by “B mass %”, the ratio of B/A is from 0.02 to 0.5. 17.The powder comprising silica-coated zinc oxide fine particles as claimedin claim 15, wherein BET specific surface area of the titanium oxide isfrom 10 to 200 m²/g.
 18. The powder comprising silica-coated zinc oxidefine particles as claimed in claim 15, wherein the average primaryparticle size of titanium oxide is 0.008 μm to 0.15 μm.
 19. The powdercomprising silica-coated zinc oxide fine particles as claimed in claim15, wherein the titanium oxide has core (a nucleus)/shell (a husk)structure, wherein the core is TiO₂-rich structure and the shell isSiO₂-rich structure.
 20. An organic polymer composition containing apowder comprising silica-coated zinc oxide fine particles as claimed inclaim 1 or 2, and a thermoplastic resin.
 21. An organic polymercomposition consisting essentially of a powder comprising silica-coatedzinc oxide fine particles as claimed in claim 1 or 2, and athermoplastic resin.
 22. The organic polymer composition as claimed inclaim 20, wherein the thermoplastic resin is selected from the groupconsisting of polyethylenes, polypropylenes, polystyrenes, polyamides,polyesters, and polycarbonates.
 23. A shape-imparted product of anorganic polymer composition as claimed in claim
 20. 24. Theshape-imparted product as claimed in claim 23, which is selected fromthe group consisting of fibers, yarns, films, tapes, hollow products,and multi-layer structures.
 25. An object comprising a shape-impartedproduct as claimed in claim 23 and selected from the group consisting ofbuilding materials for interior furnishings and exterior finish,machinery, exterior and interior decor materials for automobiles, glassproducts, electric appliances, agricultural materials, electronicapparatus, tools, tableware, bath products, toiletry products,furniture, clothing, woven fabrics, non-woven fabrics, cloth products,leather products, paper products, sporting goods, futon, containers,eyeglasses, signboards, piping, wiring, brackets, sanitary materials,automobile parts, outdoor goods such as tents, panty hose, socks,gloves, and masks.
 26. The cosmetic material comprising the powdercomprising silica-coated zinc oxide fine particles as claimed in claim 1or
 2. 27. A process for producing silica-coated zinc oxide fineparticles according to claim 1, comprising the steps of: bringing acomposition for forming silica coating into contact with raw materialzinc oxide particles whose primary particles have an average particlesize of 5 nm to 200 nm, wherein the composition for forming silicacoating contains at least the following compositions: 1) silicic acidcontaining neither an organic group nor a halogen, or a precursorcapable of producing such silicic acid, 2) water, 3) an alkali, and 4)an organic solvent, whereby surfaces of the zinc oxide particles areselectively coated with a silica coating, and subjecting the obtainedsilica-coated zinc oxide particles to a dry-format classification toreduce the number of large particles.
 28. The process according to claim27, wherein said composition for forming silica coating has awater/organic solvent ratio by volume of 0.1 to 10 and a silicon contentof 0.001 to 5 mol/L.
 29. A process for producing surface-hydrophobicizedsilica-coated zinc oxide fine particles according to claim 2, comprisingthe steps of: bringing a composition for forming silica coating intocontact with raw material zinc oxide particles whose primary particleshave an average particle size of 5 nm to 200 nm, wherein the compositionfor forming silica coating contains at least the followingcompositions: 1) silicic acid containing neither an organic group nor ahalogen, or a precursor capable of producing such silicic acid, 2)water, 3) an alkali, and 4) an organic solvent, whereby surfaces of thezinc oxide particles are selectively coated with a silica coating,subjecting the produced silica-coated zinc oxide particles to surfacetreatment with a hydrophobicity-imparting agent to obtainsurface-hydrophobicized silica-coated zinc oxide particles, andsubjecting the obtained surface-hydrophobicized silica-coated zinc oxideparticles to a dry-format classification to reduce the number of largeparticles.
 30. The process according to claim 29, wherein saidcomposition for forming silica coating has a water/organic solvent ratioby volume of 0.1 to 10 and a silicon content of 0.001 to 5 mol/L.